Of the cases reported in 2013, 1,267 were severe, defined as
causing inflammation in the brain or surrounding tissues. Cases
peaked in early September, and 90 percent of the people infected
with the virus became ill between July and September, the report
said.

Among the severe cases that caused neurological symptoms (called
"neuroinvasive" cases), more than half (51 percent) occurred in
just six states: California, Texas, Colorado, Illinois, North
Dakota and Oklahoma. The states with the highest rates of
neuroinvasive cases relative to their populations were North
Dakota, South Dakota, Nebraska and Wyoming. [ 10
Deadly Diseases That Hopped Across Species ]

West Nile virus is most commonly spread by mosquitoes. About 70
to 80 percent of people infected with the virus show no symptoms,
but about 20 percent develop a fever and other symptoms — such as
headaches, body aches and vomiting — and less than 1 percent
develop neurological illness, according to the CDC.

Because most people infected with West Nile do not develop
symptoms, or do not feel sick enough to go to the doctor, the
number of actual cases is likely much higher than the number of
reported cases, according to the CDC. Taking into account
underreporting, there may have been between 38,000 and 88,000
actual West Nile cases in the United States in 2013, the report
said.

The number of West Nile cases varies from year to year due many
factors — including the weather, number of birds that host the
virus (when it's not in mosquitoes), the abundance of mosquitoes
that spread the virus and human behavior such as the use of
insect repellent — all of which can affect when and where
outbreaks occur, the CDC said.

"This complex ecology makes it difficult to predict how many
cases of disease might occur in the future and where they will
occur," the report said.

To prevent infection with West Nile and other diseases spread by
mosquitoes and ticks, the CDC recommends that people use insect
repellant and wear protective clothing.